212 IGNEOUS PROCESSES (see later), not of complete melting With continued decompression above the point of intersection with the solidus, partial melting continues and reaches higher extents of melting for longer melting columns Thus, the volume or flux of magma generated depends on the same variables that determine whether melting occurs in the first place (lithosphere thickness, potential temperature, and water content) as well on as the flow rate of mantle source rocks through the melting region Flux Melting At subduction zones, a cold slab of old oceanic lithosphere flows down into the mantle and induces downwards corner flow of the mantle wedge above the slab and below the thick lithosphere of the overlying plate The shape of the dry solidus of peridotite and the energetics of decompression melting show that melting is to be expected under thin lithosphere, where the mantle is hot, and where the flow direction is upwards Subduction zones presumably show none of these characteristics, yet most of Earth’s most recognizable volcanoes and nearly all of the hazardous ones form above subduction zones Clearly, a great deal of melting occurs in such settings This primarily results from the introduction of water into the mantle wedge via subduction of oceanic sediments and hydrothermally altered oceanic crust At high pressures, when water can readily dissolve in magma, it acts as a flux; Figure shows that the water-saturated solidus of peridotite is several hundred degrees below the dry solidus At intermediate water contents, melting begins on a damp solidus curve between the two limits shown The process of melting by addition of water to material below its dry solidus, but at or above the wet solidus, is called flux melting, which is the second most efficient source of magmatism on Earth Subduction zone magmatism is complex, but generally, there is consensus on the essential elements The subducting slab remains relatively cold, and only the sedimentary component is thought to melt directly, creating a mobile liquid that ascends into the shallow part of the overlying mantle wedge and refreezes, modifying the composition of the wedge At somewhat greater depth, the basaltic component of the slab undergoes a series of dehydration reactions that create a water-rich fluid The stability limits of hydrous minerals in peridotite are different from those in basalt, so that as this fluid ascends out of the slab and into the immediately overlying cold part of the mantle wedge, it may also freeze in place and generate an enriched, hydrated mantle source However, with further downflow along the slab, or if the fluid can migrate far enough into the hot interior of the mantle wedge, this material crosses the hydrous solidus (region in Figure 1) and partially melts to create primary arc basalt Heating by conduction At locations remote from plate boundaries and hotspots (for example, in continental interiors), volcanism and magmatic intrusion occur, requiring a mechanism other than decompression or flux melting These locations include large rhyolitic caldera-forming systems such as Long Valley, in California Their source materials are embedded in the lithosphere (which is both too cold and too rigid for decompression melting to be effective) and, on a stable long-term geotherm, sit below even the water-saturated solidus curve Direct heating by conduction is therefore the most likely mechanism for bringing these sources into their melting range Why should conduction deliver more heat in one place than in another? The answer generally goes back to decompression melting in the underlying mantle Geochemical and geophysical evidence typically shows that, although the principal source of intraplate volcanism may be within the crust, there is often a mantle component Ponding of basaltic magma and subsequent underplating of basaltic rocks at the base of the crust are the most efficient ways to focus a large heat flow into a specific region of the crust Because basalts crystallize at temperatures above 1000 C and the rocks of the continental crust can begin melting (in the presence of water) near 700 C, it is clear that crustal melting is a likely consequence of the arrival of a large mass of basalt at the base of the crust If the basalt actually intrudes the crust and assimilates or mixes with the resulting melts of surrounding crustal rocks, the process may be described as a differentiation process (see later) affecting the basalt, as well as a melting process affecting the wallrock Magma Transport The transport of magma from regions of melting to regions of emplacement or eruption is a fundamental aspect of igneous phenomena; indeed, the outpouring of lava or explosive eruption of volcanic ash is the most obvious and hazardous manifestation of igneous activity Most of the melting processes that occur inside Earth are partial melting processes, producing a mixture of liquid and residual minerals Somehow the liquid component of this mixture is physically separated from the residue and is transported, generally to shallower depths Thus, for example, a midocean ridge basalt created by 10% melting of its mantle source may erupt as a 100% liquid, because the liquid and residue have been separated by melt